Add the ability to load only a specified list of keys from envvars

Describe the bug Migrating from other configuration systems I’d need to configure envvar_prefix to empty string ("") so I can load any environment variable I used before. But it doesn’t seem to work with dynaconf.

To Reproduce Steps to reproduce the behavior:

1. Create a basic dynaconf configuration

In config.py:

from dynaconf import Dynaconf

settings = Dynaconf(
    # This is the important line.
    envvar_prefix="",
)

print(settings.FOO)

1. Now create some settings file.

In settings.py:

FOO = 'bar'

1. Finally try running.

export SETTINGS_FILE_FOR_DYNACONF=settings
FOO='ham' python config.py

You will get:

bar

Expected behavior

The print output states:

ham

This means the value from environment variable has overridden the default value from settings.py module

Environment (please complete the following information):

• OS: Ubuntu Linux 20.04
• Python version: 3.8.5
• Dynaconf version: 3.1.2
• Frameworks in use: none.

Additional context Interestingly running in a “hacky” way changes the variable value:

_FOO='ham' python config.py

This is not what I’d expect though. The point is to modify the bare FOO variable.

[rochacbruno]

test case can be added here: https://github.com/rochacbruno/dynaconf/blob/master/tests/test_env_loader.py

and optionally a functional example in the example folder like this one https://github.com/rochacbruno/dynaconf/tree/master/example/envvar_prefix

[rochacbruno]

First you need to add the flag, any new variable on default_settings.py named in the pattern IGNORE_UNKNOWN_ENVVARS_FOR_DYNACONF will be a flag in `Dynaconf(…, ignore_unknown_envvars=True)

Here:

https://github.com/rochacbruno/dynaconf/blob/master/dynaconf/default_settings.py#L73-L222

Then you can access the variable using obj.get('IGNORE_UNKNOWN_ENVVARS_FOR_DYNACONF') here where the envvars are loaded.

https://github.com/rochacbruno/dynaconf/blob/master/dynaconf/loaders/env_loader.py#L24-L63